SoftLayer is in Dallas because The Planet, a company dating back to the late 90s, was in Dallas and SL was founded by TP's stupidly-fired COO Lance Crosby.

SL ultimately merged with/took over TP after TP had merged with EV1 (another company dating to the late 90s), Crosby continuing as CEO.

None of these were startups in the ordinary Silicon Valley sense. They were, fundamentally, managed datacenters (something their geographical location was well-suited for). The companies were built to survive and turn a profit, not achieve a massive exit in the startup lottery.